Ammonia-free, room temperature, and reusable photochemical bath for the deposition of Zn(S,O) buffer layers in Cu(In,Ga)Se₂ thin-film solar cells

Today, chemical bath deposited (CBD) Zn(S,O) is used at the industrial level in Cu(In,Ga)Se₂ solar cells technology. The state-of-the-art recipes of sulfur-based buffer layers use thiourea as sulfide ions precursor and ammonia as complexing agent. However, such formulations require high concentrations of reactants, deposition temperatures between 60°C and 80°C with the problem of ammonia losses by evaporation and large water consumption. In this work, a novel bath chemistry for Zn(S,O) buffer layer deposition is developed where the thiourea is replaced by thioacetic acid as a sulfur precursor. The use of this compound allows the photochemical growth of a dense and homogenous Zn(S,O) layer on CIGSe absorbers. The main advantages of this solution compared to classical CBD-Zn(S,O) bath are the deposition occurs at room temperature, the concentration of chemical precursors is 6 times lower, no use of a complexing agent such as ammonia, the reuse of the same bath at least for 4 consecutive times. The effect of the deposition time, the incident light power during deposition, and the successive use of the solutions on the thickness and composition of the film is discussed by means of scanning electron microscopy, and X-ray photoelectron spectroscopy analyses. The photovoltaic performance shows conversion efficiencies similar to the classical thiourea/ammonia based process.